1. Field of the Invention
The present invention relates to a dielectric material having a low dielectric loss factor for high-frequency use as employed in oscillators, antennas, filters and electronic circuit boards used in high-frequency regions such as in microwave and millimeter wave regions and, particularly, to a material for windows for transmitting high frequency such as of a high-frequency plasma-generating CVD device, a microwave output unit and an oscillator used in an apparatus for producing semiconductors, apparatus for producing liquid crystals, apparatus for producing photosensitive drums, apparatus for producing diamond films, and nuclear fusion apparatus.
2. Description of the Prior Art
In recent years, dielectric ceramics for high-frequency use have been positively used as electronic components and electronic circuit boards in high-frequency regions accompanying the development and widespread use of mobile communication equipment such as cordless telephones, simple portable telephone systems (PHS), satellite broadcast receivers, as well as high-frequency equipment and devices such as CVD apparatus for producing semiconductors and liquid crystals.
From the standpoint of enhancing performance, the dielectric materials used in high frequencies must exhibit small dielectric losses at high frequencies. Accompanying the modern trend toward adapting high-frequency technology to general applications, furthermore, study has been forwarded to utilize dielectric materials under special environmental conditions such as high-temperature and corrosive conditions. It has therefore been urged to provide a dielectric material having a low dielectric loss factor for use in high frequencies featuring excellent mechanical properties, chemical stability and high reliability.
Conventional dielectric ceramic materials of the types of BaO-TiO.sub.2, MgO-CaO-TiO.sub.2 and CaO-ZrO.sub.2 having excellent dielectric properties are not yet satisfactory in regard to their mechanical properties and chemical stability. Though a variety of attempts have been made in order to improve such properties, their strengths are about 100 MPa at the greatest. Besides, the mechanical and chemical properties vary to a large extend depending upon the Young's modulus specific to the material, mode of interatomic bonding and bonding force, leaving much room for improvements.
Ceramics having excellent mechanical and chemical properties can be represented by a sintered product of silicon nitride. According to the journal of the Japanese Association of Precision Engineering, Vol. 53, No. 5, 1987, pp. 743-748, however, the sintered product of silicon nitride used for general mechanical parts exhibits a dielectric loss factor of as large as 7.times.10.sup.-4 at a frequency of 6 GHz, and is not applicable as a dielectric material for high-frequency use. So far, silicon nitride has seldom been studied as a dielectric material.
Recently, furthermore, a microwave plasma processing apparatus has been much used chiefly in the CVD, etching and resist steps in the production of semiconductors, liquid crystals and thin films. In a high frequency generator e.g., gryotron) used for nuclear fusion, furthermore, millimeter waves in excess of 20 GHz are output and are introduced into the nuclear fusion furnace to generate a plasma of high energy.
A window transmitting high-frequency waves constituted by a material having good high-frequency permeability is used for the high-frequency introduction portion and output portion of the apparatus that generates plasma using high frequencies such as microwaves or millimeter waves. Such an introduction window and an output window (hereinafter referred to as transmitting windows), however, must have high-frequency transmission properties (low dielectric constant, low dielectric loss) as well as heat resistance to withstand a rise of temperature and a rapid change in temperature, thermal shock resistance and air-tightness for maintaining vacuum. In recent years, it has been urged to provide production apparatus featuring high productivity. In order to accomplish nuclear fusion, furthermore, a plasma of a high temperature must be produced requiring large electric power of high frequencies. Therefore, it has been demanded to provide windows having higher performance and higher reliability to meet such tendencies.
These windows have heretofore been composed of a quartz glass, alumina ceramics, single crystalline alumina (sapphire), aluminum nitride (AlN) or beryllia (BeO) having low dielectric constant, small dielectric loss and high density by giving importance to high-frequency transmission properties and air-tightness for maintaining vacuum.
However, these materials all have poor thermal shock resistance. When a large electric power of a high frequency is transmitted, furthermore, the window is locally heated giving rise to the occurrence of cracks or pinholes due to melting, resulting in a decrease in the air-tightness for maintaining vacuum.
In order to solve these problems, the surfaces of the windows have been coated with, for example, a film of TiN or a film having high thermal conductivity from the standpoint of preventing an increase in temperature on the surfaces of the windows. Or, it has been proposed, from the standpoint of increasing strength, to produce a window material using alumina ceramics as disclosed in, for example, Japanese Laid-Open Patent Publication No. 345527/1994 or using an alumina-zirconia composite material as disclosed in Japanese Laid-Open Patent Publication No. 280976/1992. These ceramics, however, have a thermal shock resistance of as low as about 200 to 300.degree. C., which is not sufficient from the standpoint of reliability. Besides, cracks easily develop in the windows impairing mechanical reliability and making it difficult to dear with large electric power at high frequencies.
There has further been known a silicon nitride (Si.sub.3 N.sub.4) ceramics materials used for which exhibits far superior heat resistance, thermal shock resistances and mechanical properties to those of the above-mentioned ceramics. However, the silicon nitride has been used as structural components but has not been studied in regard to being used as a window material for introducing high frequencies by reason of having a high dielectric loss.
In order to obtain a material having a high strength and a low dielectric loss factor in a high-frequency band, the present inventors have forwarded the study extensively to decrease dielectric loss by using a sintered product of silicon nitride that has excellent mechanical and chemical properties but a large dielectric loss factor. As a result, the inventors have discovered the fact that the dielectric loss factor at 10 GHz can be decreased down to not larger than 5.times.10.sup.-4 if the content of aluminum that is inevitably contained as impurities in the sintered product of silicon nitride is suppressed to be smaller than a predetermined amount and that there can be obtained an excellent material having a small dielectric loss factor that is suited for use in high frequencies, and have thus arrived at the present invention.
The inventors have further discovered the fact that particularly excellent properties are obtained when a material of the present invention is used and, particularly, when a material of the invention having a relative density of not smaller than 97% and a dielectric loss factor of not larger than 5.times.10.sup.-4 is used as a window for high frequencies such as in the aforementioned CVD apparatus, microwave output unit and oscillator.